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Objective

This project builds on previous work aimed at investigating the ignition of small methyl esters as biodiesel surrogates. Methyl formate, methyl butanoate and n-heptane in oxygen/argon mixtures are studied in a shock tube facility.  The experimental data are intended to be used to advance the chemical kinetic modelling of biodiesel surrogates. The ignition behaviour of n-heptane, a conventional diesel surrogate, has also been studied and the experimental data are compared to those of methyl butanoate. Chemical kinetic models are analysed in order to establish the differences and common features in the combustion modeling of methyl esters and n-alkanes.

Approach

Research on the ignition of biodiesel surrogate fuel has been carried out at the Alternative Fuels Laboratory at McGill University. In the framework of this research project, a shock tube facility has been developed. The driver and driven sections of the shock tube are respectively 3.0 and 4.2 m with an internal diameter of 5.0 cm. The combustible mixture is rapidly subjected to test conditions of high temperature and pressure by means of a shock wave. The shock velocity is measured by means of fast-response pressure transducers, and is in turn used in the shock equations to determine the post-reflected shock temperature of the mixture. By means of a photodiode, the light emission (attributed to CH radical emission at 430 nm) is used to determine the ignition delay time. To assess the performance of the facility, it has been validated against n-heptane and iso-octane ignition data from the literature. Chemical kinetic simulations are carried out using the software package CANTERA. These simulations are compared to experimental data and the underlying chemical reaction pathways are highlighted through analyses of the mechanisms. Suggestions are made regarding the optimization of the kinetic mechanisms [1,4,5]. Comparing the ignition behaviour of methyl formate and methyl butanoate to selected n-alkanes reveals the similarities and differences in methyl ester and n-alkane combustion chemistry.

Accomplishments

1.      Characterisation of shock tube in the framework of an honour thesis. The undergraduate student trained during this project is now completing a master program in our research group. The PhD working on this project continues to gain skills in research and technical writing and presentation of research outcomes.

2.      Methyl formate ignition study has been completed. The results were presented at the annual meeting of the Canadian Section of the Combustion Institute. Focus of the study was the investigation of the effect of pressure, equivalence ratio and argon dilution on methyl formate ignition. Further studies involving comparison with model predictions and analysis of mechanisms have been carried out. This work has been accepted by the journal Energy & Fuels pending minor revisions. It has been observed that methyl formate ignites more readily than methane but less than ethane at the same temperature, pressure, equivalence ratio and argon/oxygen ratio. Analyses of the published mechanisms and literature suggest that unimolecular decomposition of methyl formate is the main pathway for methyl formate oxidation.

3.      Methyl butanoate ignition studies have been carried out. The first results of an extended parameter study, correlation and comparison with model predictions were presented at the 22^nd International Colloquium on the Dynamics of Explosions and Reactive Systems in Minsk, Belarus in July of 2009. Further studies have focused on the comparative ignition behaviour of methyl butanoate and n-heptane as surrogates of biodiesel and diesel fuels. It has been observed that at the same pressure and argon/oxygen ratio, the high temperature ignition behaviour of both fuels is comparable. This finding is particularly interesting to chemical kinetic model developers. Recent developments in chemical kinetic modelling show that higher n-alkanes/air mixtures from n-heptane to n-hexadecane display similar ignition delays [6]. It has been suggested that mixtures of higher methyl esters with air would have comparable ignition delays with higher alkanes. The comparative study of methyl butanoate and n-heptane was intended to provide further insight on this relative behaviour. Chemical kinetic models are expected to be capable of predicting a wide range of combustion properties. As a result, most existing mechanisms are in a constant state of optimization. For this reason experimental data such as these as well as knowledge of the relative ignition behaviour of fuel surrogates will assist in mechanism development, optimization and validation.

This research project has contributed towards understanding methyl ester ignition as biodiesel surrogates. The results of the methyl formate ignition have shown that even in the simplest methyl ester, increased reactivity compared to methane is observed. The comparative study of methyl butanoate and n-heptane has led to a better understanding of the similarities in the combustion of methyl esters and n-alkanes. The effects of pressures and equivalence ratio have been investigated and found to be comparable within the limit of accuracies of shock ignition data. These observations point to the importance of focusing on modelling the differences in other combustion properties such NOx and soot formation while comparing ignition behaviour at all stages of mechanism development and optimization.

Future work

·        Comparative studies of methyl ester ignition. Methyl formate, acetate, propanoate and butanoate will be studied to establish the influence of the length of the alkyl group on methyl ester ignition.

·        Comparative studies of methyl butanoate, butane, butylaldehyde, butanone and butanol to establish the influence of the methoxy group on ester ignition.

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[2.]       Metcalfe, W.K, Dooley, S., Curran, H.J., Simmie, J.M., El-Nahas, A.M. and Navarro, V.M.  J. Phys. Chem. A 2007, 111, 4001-4014.

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[6.]       Westbrook, C.K.; Pitz, W.J.; Herbinet, O.; Curran, H.J.; Silke, E.J. Combustion and Flame

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